Radiation exposure to the entire body can be a severe detriment to human health. Studies can be conducted in non-human primates and small animals, but the extrapolation of this data to the human condition does not cover the whole story. Obviously, direct experimental studies on humans is not an option. Historically severe cases of acute and chronic total body irradiation (TBI) in humans are studied by analyzing dramatic events, such as reactor accidents and nuclear bomb bursts, and medical therapeutic treatments.
The cases of leukemia induced by different exposure events have been documented. [1] The most striking data is that which comes from the population of survivors following the nuclear bomb blasts over Japan in WWII. Proximity of exposure and leukemia incidence data compiled from the National Academy of Sciences "Atomic Bomb Casualty Commission" and presented in the E. B. Lewis paper is summarized in Table 1.
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| Table 1: Combined incidence of leukemia from Hiroshima and Nagasaki bomb bursts between 1948 and 1955. [1] |
Alternatively, TBI can be harnessed in a way that is beneficial toward human health. Some cases include the use of radiotherapy units that deliver megavoltage photon beams to patients in preparation for bone marrow transplant and the treatment of diseases such as leukemia, aplastic anemia, lymphoma, multiple myeloma, autoimmune diseases, and inborn errors in metabolism. [2] Giving TBI to a patient enhances their ability to accept bone marrow by suppressing the immune systems viability, ie. killing leukocytes. TBI can also be used in conjunction with chemotherapy to further destroy tumor cells and enhance the effectiveness of the therapy.
In both scenarios described here, it is the interaction of high energy photons and/or particles with human cells that determines a biological response. At around 1000 m the absorbed dose for individuals was around 4.5 Gy and 8.7 Gy for Hiroshima and Nagasaki respectively and at 2000 m the doses were around 0.08 Gy and 0.14 Gy. [3] In contrast, the typical TBI treatment for immune suppression involves multiple fractions, on the order of 5 to 8 on subsequent days, with a cumulative dose of around 13 Gy. [4] It is interesting to note that the total dose for TBI is greater than the 1000 m dose for both bomb bursts and that the single exposure from these events was likely 100% fatal. By spreading out the dose over multiple days, the body has time to repair DNA damage before cell death becomes rampant and to repopulate intact tissue. [5]
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[1] E. B. Lewis, "Leukemia and Ionizing Radiation" Science 125, 965 (1957).
[2] F. M. Kahn, The Physics of Radiation Therapy, 3rd Ed. (Lippincott Williams & Wilkins, 2003), pp. 455-463.
[3] T. Imanaka, "Casualties and Radiation Dosimetry of the Atomic Bombings on Hiroshima and Nagasaki," in Radiation Risk Estimates in Normal and Emergency Situations, ed. by A. A. Signa and M. Durante (Springer, 2006).
[4] J.R. Greig et al., "Head and Neck Compensation for Total Body Irradiation Using opposed Laterals," Medical Dosimetry 23, 11 (1998).
[5] E. L. Alpen, Radiation Biophysics, 2nd Ed. (Academic Press, 1998), pp. 293-305.